What is nut allergy and what causes it?

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What is nut allergy?

An allergy occurs when your body's immune system, which normally fights infection, over-reacts to a substance called an allergen. Most allergens are not obviously harmful and they have no effect on people who are not allergic to them. Allergic reactions to allergens can vary from mild to life-threatening.

Both peanuts and tree nuts (for example, walnuts, hazelnuts, almonds, cashews, pecans, brazils and pistachios) can act as allergens, and can cause an allergic reaction in some people. When you come into contact with something that you are allergic to (an allergen), a group of cells in your body called mast cells release a substance called histamine. Histamine causes the tiny blood vessels in the tissues of your body to leak fluid which causes the tissues to swell. This results in a number of different symptoms.

Strictly speaking, peanuts are not nuts - they grow underground whereas other nuts grow on trees. The word nut in this article can mean either tree nuts or peanuts.

What causes nut allergy?

If you are allergic to nuts, when you first come into contact with nuts your immune system reacts and prepares to fight. However, you don't get any symptoms of a reaction. It is only when you come into contact with nuts for a second time that a full allergic reaction happens. Most children who are allergic to nuts have the symptoms of an allergic reaction when they appear to be exposed to nuts for the first time. However, this is probably not their first exposure, but their second. They may already have come into contact with nuts through their mother - either whilst they were in the womb or through breast milk if they were breastfed.

Most people with nut allergy react after contact with small amounts (less than one nut) and some people may react to trace amounts. This means that you don't always have to eat nuts to have a reaction. A few people are so sensitive to nut allergens that a tiny amount on their lips, or even standing next to someone eating peanuts, can be enough to start a reaction.

There are lots of different allergens but nuts cause some of the strongest and most severe reactions. Doctors don't yet know why this is.